Docs feedback: Add housekeeping to the measures to reduce the repository size

Link: https://docs.gitlab.com/ee/user/project/repository/reducing_the_repo_size_using_git.html

I recently ran into an issue that my repository was full (reached 10 GB on GitLab; no LFS btw). That seemed a bit strange to me and I checked locally, which resulted in a size of about 5 GB and about 4 GB of garbage. After running garbage collection locally and having the size limit temporarily set to a higher amount, I synced the repo again and now it showed about 5 GB size on GitLab.

Before that I read the documentation about how to reduce the repo size. Unfortunately garbage as a possible reason was not mentioned there.

As I learned just now, there is also GitLab housekeeping: https://docs.gitlab.com/ee/administration/housekeeping.html

I guess that could have helped me also instead of locally looking for the source of that problem and bothering the support to raise the limit temporarily.

I suggest to add housekeeping to the measures of how to reduce the repo size and/or to the size limit documentation: https://docs.gitlab.com/ee/user/admin_area/settings/account_and_limit_settings.html